In canon... Becky! j/k. No one. I feel like everyone has been bad for him or not right for him in one way or another. If pressed to choose someone I guess I'd say the werewolf girl.
In Marina, Sasha. She's kind to him which is really not something he got a lot of, and she is a goddess so she would understand his world too. Or at least understand more than some. She's also the goddess of wisdom and war and well... right up his alley.
I could ship him with Lacus, Buffy, and Faith too.
Top 3 people Castiel finds interesting in Marina (not Sam and Dean)?